Lithium secondary cells have been actively used as high-performance secondary cells. A lithium secondary cell is provided with a positive electrode having a positive electrode active material made of a lithium-containing complex oxide, a negative electrode having a negative electrode active material capable of absorbing and releasing lithium ions, a separator interposed between the positive electrode and negative electrode, and a nonaqueous electrode impregnated in the positive electrode, negative electrode and separator. When the lithium secondary cell is produced, the positive electrode, negative electrode, and separator are assembled and impregnated with the nonaqueous electrolyte, followed by charging.
When the lithium secondary cell is produced, metal impurities such as copper or iron can be admixed from the outside. Where those metal impurities are dissolved in the nonaqueous electrolyte during charging and precipitate in a concentrated manner on the negative electrode, the precipitates can break through the separator and reach the positive electrode, thereby causing short circuiting. Accordingly, a variety of techniques have been used to prevent short circuiting caused by metal impurities.
Patent Literature 1 describes the feature of applying 100 to 10,000 times a pulse voltage with a period of 1 to 100 ms so that the closed-circuit potential of a positive electrode becomes 3.8 V to 4.2 V with respect to the lithium dissolution and precipitation potential (in other words, the redox potential) and the closed-circuit potential of the positive electrode in a state with the open cell case becomes 4.4 V to 4.5 V with respect to the lithium dissolution and precipitation potential. Patent Literature 1 indicates that the repeated application of such a low pulse voltage makes it possible to dissolve effectively the residual alkalis present on the surface of a nickel-containing positive electrode active material, and that the expansion of the cell and the increase in internal resistance can be inhibited by sealing the cell case after releasing the gases generated by the dissolution.
Patent Literature 2 describes performing the initial charging in a state in which the electric potential E1 of a negative electrode is maintained within a range of 2.5 V<E1<3.2 V. Patent Literature 3 describes the feature of charging at least once for 1 h, then discharging till the negative electrode potential becomes 2.0 V to 3.35 V with respect to the lithium redox potential, and allowing to stand for 3 min or more in this state. Patent Literature 4 describes the feature of mixing an additive that can be reduced at a negative electrode at an electric potential equal to or higher than 1.5 V and charging only a positive electrode in the initial charging. Patent Literature 5 describes the technique for efficiently removing metal particles when purifying a carbon material that can be used as a negative electrode active material. Patent Literature 6 describes the feature of inhibiting the internal short circuit by setting the admixed amount of a transition metal element other than the transition metal element constituting a positive electrode active material to a predetermined value or below. Patent Literature 7 describes the feature of charging to 0.01% to 0.1% of the cell capacity during the initial charging and then providing a standing time of 1 h to 48 h.